Several molybdenum compounds have been used as lubricating oil additives. For example, molybdenum disulfide in dispersed form is an effective lubricating oil additive. Another compound which has been used is molybdenumdioxy dialkyldithiocarbamate described in U.S. Pat. No. 3,419,589. Several molybdenum dialkyldithiophosphate complexes are described in U.S. Pat. Nos. 3,068,259; 3,400,140 and 3,402,188.
Diesel engines are well known for their long endurance under most severe conditions. Because of this they have found favor for use in heavy duty trucks and locomotives. Although diesel engines have seen limited use in light duty automotive application, it is only recently that such use has begun to increase sharply. This is due to industry attempts to achieve increased fuel economy. In general, these light duty automotive diesel engines are not as heavily constructed as prior heavy duty engines and less expensive metals and metal alloys are used. This has brought about a wear problem in light duty automotive diesel engines that was not of such significance in heavy duty engines.
The wear problem appears to be due mainly to blow-by carbon soot which accumulates in the crankcase. This soot either causes wear or serves to negate the effect of additives such as zinc dihydrocarbyldithiophosphates which are customarily added to inhibit wear. In fact, in tests it has been found that zinc dihydrocarbyldithiophosphates, rather than acting as a wear inhibitor, can, in the presence of carbon soot, cause an increase in wear.